This invention relates to a titanate- or stannate-based reference electrode for electrolytic cells having an ion-conducting solid electrolyte, to sensors comprising the reference electrode of the invention and to use of the reference electrode for analyzing gases.
The use of electrolytic cells having a solid electrolyte as gas sensors, especially as oxygen sensors, has been known for a long time, However, it is difficult to realize reference electrodes of practical applicability. The use of cation-conducting solid electrolytes, for example a sodium-ion conductor, necessitates maintaining a defined sodium potential over long periods of time. According to prior art, this is achieved by making use of metallic sodium, which is liquid and highly reactive at the operating temperatures--frequently above 500.degree. C.--of the sensor. The construction of such an electrode thus involves substantial difficulties, namely in insulating the electrode spaces hermetically from one another and from the surroundings. Reactions of the molten sodium with the insulating material or with other components of the electrolytic cell result in the stability of the sensor signal being impaired or in the sensor being totally destroyed with time.
In the DE-A-41 12 301.8 the alternative use of a reference electrode is suggested which contains an alkali-metal compound, in particular a sodium compound in multinary multi-phase equilibrium (e.g. binary Na/metal compounds or ternary Na/metal/oxide compounds). Examples of such reference electrodes having binary sodium/metal compounds are Na--Sb or Na--Bi, examples of such reference electrodes having ternary sodium/metal/oxide compounds are Na--Co-oxide or Na--Ni-oxide. However, on account of the toxicity of the heavy metal compounds used, production of these reference electrode systems is problematical.
It is also known that the metal activity brought about at the phase boundary between a solid electrolyte and a precious metal adhering thereto fulfills the function of a reference system (cf. Saito and Maruyama, Solid State Ionics 28-30 (1988), 1644). Here there is often the danger, however, that due to the inherently incomplete separation of reference and measuring electrodes the reference will react with the measuring medium, for example CO.sub.2 and O.sub.2. It is then only a matter of time until the cell voltage of a sensor of this type decreases to 0 and the reference electrode loses its functionality (cf. Maruyama et al., Solid State Ionics 23 (1987), 107).
Maier and Warhus (J. Chem. Thermodynamics 18 (1986), 309-316) describe thermodynamic studies carried out on Na.sub.2 ZrO.sub.3 using electrochemical measurements. To this end an electrochemical cell was prepared which contained as reference electrode a mixture of sodium zirconate, zirconium dioxide and metallic gold. This reference electrode makes contact by way of a sodium-ion conductor with a measuring electrode containing a mixture of sodium carbonate and metallic gold. This electrochemical cell can also be used as a CO.sub.2 sensor (cf. e.g. Maier, in: Science and Technology of Fast Ion Conductors, publ. H. L. Tuller and M. Balkanski, Plenum Press, New York (1989), pp. 299; Maier, Solid State Ionics 62 (1993, 105-111). This zirconate-based sensor has the advantage that the measuring electrode and the reference electrode in the cell can be exposed to the same CO.sub.2 partial pressure, which means that the signal obtained depends only on the CO.sub.2 and not on the O.sub.2 partial pressure. It is therefore not necessary to seal the reference electrode.